Wahab Abdul, Mahiuddin Sekh, Hefter Glenn, Kunz Werner, Minofar Babak, Jungwirth Pavel
Material Science Division, Regional Research Laboratory, Jorhat--785 006, Assam, India.
J Phys Chem B. 2005 Dec 22;109(50):24108-20. doi: 10.1021/jp053344q.
The ultrasonic velocities, densities, viscosities, and electrical conductivities of aqueous solutions of magnesium nitrate and magnesium acetate have been measured from dilute to saturation concentrations at 0 < or = t/degrees C < or = 50. The temperature derivative of the isentropic compressibility, kappa(s), became zero at 2.28 and 2.90 mol kg(-1) for Mg(OAc)2 and Mg(NO3)2 solutions, respectively, at 25 degrees C. The total hydration numbers of the dissolved ions were estimated to be, respectively, 24.3 and 19.2 at these concentrations. Differences in kappa(s) for various M2+ salts, using the present and literature data, correlated with reported M2+-OH2 bond lengths and to a lesser extent with cationic charge densities (ionic radii). The influence of anions on kappa(s) appears to follow the Hofmeister series and also correlates approximately with the anionic charge density. Substantial differences between Mg(OAc)2(aq) and Mg(NO3)2(aq) occur with respect to their structural relaxation times (derived from compressibility and viscosity data) and their electrical conductivities. These differences were attributed to a much greater ion association in Mg(OAc)2 solutions. Raman spectra recorded at 28 degrees C confirmed the presence of various types of contact ion pairs including mono- and bidentate complexes in Mg(OAc)2(aq). In Mg(NO3)2(aq), only noncontact ion pairs appear to be formed even at high concentrations. The experimental results are supported by molecular dynamics simulations, which also reveal the much stronger tendency of OAc- compared to NO3- to associate with Mg2+ in aqueous solutions. The simulations also allow an evaluation of the ion-ion and ion-water radial distribution functions and cumulative sums and provide a molecular picture of ion hydration in Mg(OAc)2(aq) and Mg(NO3)2(aq) at varying concentrations.
在0≤t/℃≤50的条件下，对硝酸镁和醋酸镁水溶液从稀溶液到饱和溶液的超声速度、密度、粘度和电导率进行了测量。在25℃时，醋酸镁和硝酸镁溶液的等熵压缩率κ(s)的温度导数分别在2.28和2.90 mol kg⁻¹时变为零。在这些浓度下，溶解离子的总水合数估计分别为24.3和19.2。利用本文数据和文献数据，各种M²⁺盐的κ(s)差异与报道的M²⁺-OH₂键长相关，在较小程度上与阳离子电荷密度（离子半径）相关。阴离子对κ(s)的影响似乎遵循霍夫迈斯特序列，并且也大致与阴离子电荷密度相关。醋酸镁水溶液和硝酸镁水溶液在结构弛豫时间（由压缩率和粘度数据得出）和电导率方面存在显著差异。这些差异归因于醋酸镁溶液中离子缔合程度大得多。在28℃记录的拉曼光谱证实了醋酸镁水溶液中存在各种类型的接触离子对，包括单齿和双齿配合物。在硝酸镁水溶液中，即使在高浓度下似乎也只形成非接触离子对。分子动力学模拟支持了实验结果，该模拟还揭示了在水溶液中，醋酸根离子与镁离子缔合的倾向比硝酸根离子强得多。模拟还允许评估离子-离子和离子-水的径向分布函数以及累积和，并提供了不同浓度下醋酸镁水溶液和硝酸镁水溶液中离子水合的分子图像。
